There were small groups of Christians immediately after the death of Christ which continued to grow stronger. By 313 AD Constantine made Christianity an official religion. In fact by the time Mohammed made it on the scene around 630 AD he is writing of the “Peoples of the Book” and he is referring to the Jews and the Christians (New and Old Testament and Apocryphal writings). A lot of the Koran is taken out of the Old Testament and other Jewish writings. The Prophet Mohammed claims to be the successive prophet to Abraham, David and Jesus.

Charlemagne is about AD 800 and the crusades are about 1000 AD. So by the time the crusades start there was a long history of Christianity in the Holy Land though by that time the countries that had Christianity as its predominate religion was Italy and the Western countries where Rome had invaded.

There is a lot more on Wikki if you want to brush up on religious history quickly without spending too much time. If you want to read a marvelous book on the Prophet Mohammed read Karen Armstrong’s book.